My present invention relates to a process for the production of sliding elements such as bearings, seals, antifriction elements and the like. The invention also relates to a multilayer material for such applications.
Sliding elements should be understood to include all types of bearing liners as well, inter alia, as pistons and piston rings. Overlays of multilayer materials for sliding elements are generally produced by electroplating in appropriate electrolyte baths. There have hitherto generally been used for this purpose fluoroborate-containing baths, which have a number of disadvantages.
Deposits from fluoroboric acid-based baths are unstable at the temperature at which crankshaft bearings operate in internal combustion engines. The main alloying element in the lead-based overlay, which may be tin or indium, diffuses at engine temperature into the underlying copper-lead lining, resulting in a loss of tin or indium from the overlay and a corresponding reduction in the resistance of the overlay to wear, fatigue cracking and corrosion. Diffusion into the copper-lead lining may be prevented by the incorporation of an electro-deposited nickel xe2x80x9cdamxe2x80x9d between the copper-lead lining and the overlay, but diffusion towards the nickel still occurs, resulting in the growth of a nickel-tin compound such as NiSn on the surface of the nickel dam. A degree of loss of the alloying element from the overlay thus occurs in spite of the presence of the overlay results. However, the extent of the deterioration is not as great when a nickel dam is present as when it is not, and for that reason a nickel dam is generally incorporated in overlay-plated copper-lead crankshaft bearings.
The significant discovery disclosed in DE 195 45 427 A1 is that lead-based overlays may be deposited from an alkyl-sulfonic bath which show the delirious diffusion behavior described above to a much lesser extent than overlays from a fluoroboric acid bath. The reduction in diffusion is such that the nickel dam may be omitted, resulting in a simplification of the plating process, while at the same time an improvement in the bearing performance of the overlay is obtained. In particular the resistance of the overlay to corrosion is improved, and the wear incurred during engine operation is reduced. The experimental results demonstrating these advantages have been described in xe2x80x9cNew Bath Formulation for Electrodeposition of Improved Corrosion and Wear-Resistant Sliding Layers for Engine Bearingsxe2x80x9d by Grxc3xcinthaler, Hun, Muiller, Staschko and Toth, SAE Paper 960964, Feb. 26-29, 1996.
Although hardness was improved by 20% compared to conventional sliding layers, wear resistance was not yet satisfactory.
It is therefore an object of the invention to so improve the process known from DE 195 45 427 as to further increase the wear resistance of the sliding layer.
It is also an object of the invention to create an improved layer compound material.
Still another object is to provide a method of making an improved bearing material, especially for crankshaft and like bearings, whereby the drawbacks of earlier systems are obviated.
The object is achieved by a process whereby, after application of the galvanic running layer, a heat treatment is performed in the temperature range 150xc2x0 C. to 200xc2x0 C. for a period of 1-100 h.
It has been found that the heat treatment results in diffusion and coming together of the copper and tin components, so that particles of the intermetallic phase Cu6Sn5 are generated. Due to these Cu6Sn5 particles the wear of the sliding layer is reduced by approximately 50%.
The heat treatment is preferably executed in air.
According to a further embodiment, the heat treatment may also be executed in oil, whereby paraffin oil and silicone oil are particularly suitable.
The heat treatment is preferably performed in a temperature range between 170xc2x0 and 185xc2x0 C.
The heat treatment is preferably performed for a period of 4-12 h.
The process known from DE 195 45 427 is especially well suited to generate both tin and copper in high concentrations. It was found that the content of Cu6Sn5 particles is greater, the higher the tin and copper contents are. To further improve wear-resistance, the running layer of 12-16 weight-% tin and 7-11 weight-% copper and remainder lead is preferably applied.
The layer material for sliding members shows 15-25 weight-% Cu6Sn5 particles in the sliding layer.
The lead-bronze layer and a nickel layer are preferably provided between the sliding layer and the support layer.